K. Schuler, M. Younis, R. Lenz and W. Wiesbeck describe in “Array design for automotive digital beamforming radar system”, Radar Conference, 2005 IEEE International, May 9-12, 2005, pages 435-440, a method and an apparatus for monitoring an area by means of the technique of digital beamforming, in which an antenna array having a plurality of transmitters and a plurality of receivers is used. The monitored area is imaged by means of radar, wherein in the dimensions distance and angle the spatial position of objects in the monitored area is determined. The angle resolution results from the phase differences of the signals received by different receivers at the same time. The resolution increases with the number of combinations of transmitter and receiver for which an evaluable receive signal is received. In the method proposed in this publication, a plurality of transmitters are used, which transmit their transmit signals in chronological succession to the same area. By means of each separate digitalization of signals received by the individual receivers for each of the transmitters, with M transmitters and N receivers, M·N receive signals are available for the digital beamforming. Such a multi-transmitter/multi-receiver-system, compared to other known radar systems, offers the advantage of an angle resolution which is independent of the bandwidth. These systems play an increasingly important role in automobile short-range sensor systems.
Systems with a plurality of transmitters and a plurality of receivers offer already, on the one hand, with a small number of transmitters and receivers a high resolution, but on the other hand, only a moderate dynamic range.
For conventional antenna systems with only one transmitter and a plurality of receivers for radar monitoring, it is known to increase the dynamic range of the angle measurement by means of an amplitude weighting. Such amplitude weighting, however, achieves the desired result only if a high number of receivers is available. Only then the necessary degrees of freedom are available to increase the dynamic range of the measurement considerably. This, however, does not apply for the systems with a plurality of transmitters and a plurality of receivers considered here, because those, in general, are aimed to offer a good resolution with a small number of elements, in particular with less than four transmitters and less than four receivers. An application of amplitude weighting to the small number of transmitters or the small number of receivers can not result in a noticeable increase of the dynamic range in such a case.
It is an object of the present invention to provide a method for the operation of an antenna group comprising a plurality of transmitters and a plurality of receivers, which allows a substantial increase of the dynamic range. Furthermore, an apparatus is to be provided, by means of which the method can be performed.